The aims of the present study were to evaluate: i.) mixtures of spring barley (Hordeum vulgare L.) by combining cultivars differing in plant height or time of maturity, and ii) mixtures of cultivars of cocksfoot (Dactylis glomerata L.) and timothy (Phleum pratense L.) based on differences in maturity group. Experiments were conducted on mixtures of cultivars of spring barley and on mixtures of cocksfoot and timothy. The grain yields of all the mixtures tested in 1985 were similar. Of 4 mixtures tested, Midas/Triumph and Kym/Klaxon outyielded (though not significantly) their respective highest yielding pure stand component. Within mixtures, the taller or the earlier maturing component was more aggressive than the other component. All four mixtures produced relative yield totals (RYT) greater than 1.0. The highest RYT's came from mixing cultivars of different maturities. In the second experiment, in 1986, Triumph outproduced both Doublet and Klaxon, having more ears and grains per M2. The mixtures did not vary in grain yield from the mean of their pure stand components. In mixtures, the taller component was generally more dominant. In contrast to the previous year, all three mixtures tested in 1986 had yields approximately double those of the previous year, and RYT's less than 1.00. In the third experiment, application of fertiliser had a significant effect on the grain yield of pure stands and mixtures. At low fertilty, Triumph outproduced Doublet in both number of grains per area and grain yield, though at moderate fertility the opposite was true. In mixture, Triumph outcompeted Doublet at both low and moderate fertility levels. It produced more ears and grains per m2 and thus higher grain yields. The relative yields of Triumph were higher than those of Doublet. Similar results were found in glasshouse experiments. An experiment was set up in 1985, and ran over for 3 years, to investigate the performance of mixtures of early and late cultivars of cocksfoot and timothy. In the establishment year, there was no difference between total dry matter yield of full density pure stands of cocksfoot and timothy, though cocksfoot produced more tillers but less total harvestable leaf area than timothy. In the second year, full density pure stands of timothy produced higher total dry matter, mean number of tillers and total harvestable leaf area than full density pure stands of cocksfoot. In the third year (the only year when fertiliser was applied), full density pure stands of cocksfoot produced higher total dry matter, mean number of tillers and total harvestable leaf area than full density pure stands of timothy. Arguably, the best overall yield came from early cocksfoot/late timothy because it had one very high yielding year and in no year was RYT less than 1.0. The results from all the experiments suggest that mixtures can in some circumstances outyield the midmonoculture yields of their components. This occurs when (i) the components are of contrasting types ( e.g. in morphology and growth pattern) and (ii) when pure stand yields are inherently low. This reinforces the opinion that mixtures may be of greatest benefit in low input agricultural systems.